This application claims the priority of German patent document 100 218 14.8, filed May 4, 2001,
The invention relates to a steering system for a motor vehicle.
A steering system of this kind is known from DE 196 07 028 C1 and is designed in such a way that it allows steer-by-wire operation, with a vehicle steering wheel being effectively connected to steerable vehicle wheels by a control system without a positive mechanical connection between the steering wheel and steerable vehicle wheels. A steering system of this kind is provided with a hand-torque actuator, which is connected to the steering wheel in steer-by-wire mode to simulate a steering resistance and/or a restoring torque. The hand-torque actuator can be used to provide haptic feedback at the steering wheel even in steer-by-wire modexe2x80x94as in a conventional vehicle steering system with a continuous mechanical driving link between the steering wheel and steerable vehicle wheelsxe2x80x94this feedback representing a restoring force and a steering resistance. A hand-torque actuator of this kind can likewise be used to simulate steering and disturbing forces actually acting at the steerable vehicle wheels.
Simulation of a steering resistance is of considerable importance for steering feel in steer-by-wire mode.
If the hand-torque actuator fails on a curve during a journey in steer-by-wire mode, the steering resistance simulated by the hand-torque actuator suddenly disappears. Since this steering resistance artificially produced by the hand-torque actuator is in equilibrium with the hand force introduced into the steering wheel by the driver, at least during a constant cornering movement, failure of the hand-torque actuator leads to a sudden disappearance of the steering resistance, with the result that only the hand force of the driver is acting. This can lead to an uncontrolled steering movement.
In order to avoid uncontrolled steering movement in the event of the hand-torque actuator failing, the actuator can have a redundant construction, i.e. the significant components of the hand-torque actuator are duplicated. Apart from the associated complexity and costs, there is also no room to install a redundant hand-torque actuator in certain types of vehicles.
The present invention is concerned with the problem of specifying an embodiment of a steering system of the above discuss type stated at the outset which can ensure adequate vehicle safety even if the hand-torque actuator fails in steer-by-wire mode. Additionally, the steering system should be relatively inexpensive and should require only a relatively small amount of installation space.
The invention is based on the general idea of damping adjusting movements of the steering handle initiated by the driver, at least if the hand-torque actuator fails.
The provision of damping means eliminates the need to fit a redundant hand-torque actuator, thereby making it possible to save on costs, on the effort involved in assembly and on installation space. The invention exploits the realization that it is sufficient to guarantee vehicle safety in the event of the hand-torque actuator failing if the adjusting movements of the steering handle by the driver are dampened in steer-by-wire operation.
It is clear that xe2x80x9cfailurexe2x80x9d of the hand-torque actuator is understood to mean only a malfunction of the kind where the steering handle is not locked by the hand-torque actuator.
In the present invention, the term xe2x80x9cdampingxe2x80x9d is based on the following convention: while a spring action produces displacement-dependent counterforces and countertorques, damping produces speed-dependent counterforces and countertorques. In the present case, this means that relatively rapid adjusting movements of the steering handle by the driver are counteracted by relatively large damping torques while, correspondingly, relatively slow adjusting movements by the driver are counteracted by relatively small damping torques. Since the speed of the adjusting movements by the driver is correlated with the magnitude of the hand torque introduced into the steering handle by the driver, the damping torque counteracting the adjusting movement by the driver depends on the magnitude of the hand torque introduced by the driver.
According to a preferred embodiment of the present invention, the hand-torque actuator can include an electric motor, which is connected to the steering handle in such a way that adjusting movements of the steering handle are associated with relative rotation between a rotor and a stator of the electric motor. The damping means can then have a circuit arrangement that switches windings of the electric motor in such a way that voltages induced by relative rotation between the rotor and the stator produce damping torques that counteract the relative rotation. Here, the effect of the circuit arrangement is that the electric motor operates as a generator if the hand-torque actuator fails. By virtue of this measure, the damping function, in effect, integrated into the hand-torque actuator or electric motor. Such a circuit arrangement requires only a small amount of installation space and is generally inexpensive. Moreover, the circuit arrangement can be mounted directly on the hand-torque actuator or electric motor and can therefore be installed in the electric motor at the manufacturers. It is thus possible to provide a hand-torque actuator with an integral damping function which can be activated for emergency operation of the hand-torque actuator in the event of relevant malfunctions. The integral construction furthermore simplifies assembly without giving rise to high additional costs for emergency operation, requirements of the hand-torque actuator.
In a development of the invention, it is also possible for the circuit arrangement to be designed in such a way that the damping depends on additional parameters, e.g. vehicle speed. At higher vehicle speeds, for example, the damping should be greater than at low vehicle speeds. In particular, the damping can also have a value of zero while the vehicle is being manoeuvred.
According to another embodiment, the damping means can have at least one mechanically operating damper member, which is coupled to the steering handle. A damper member of this kind can, for example, be coupled permanently to the steering handle with its damping action being compensated for in normal operation of the hand-torque actuator by appropriate control of the hand-torque actuator. Another possibility is a damper member that can be connected when required and is therefore coupled to the steering handle only if the hand-torque actuator fails. A damper member of this kind can produce the required damping by mechanical friction, for example. As an alternative, a damper member of this kind can also be designed in such a way that the adjusting movements of the steering handle by the driver drive or displace a fluid in the damper member, the damper member producing the desired damping by restricting the flow of this fluid.
In a preferred development of the steering system according to the invention, a spring device preload the steering handle into a central position. This spring device can be used to apply a restoring torque to the steering handle.
It is self-evident that the features mentioned above and those that will be mentioned below can be employed not only in the respectively indicated combination but also in different combinations or in isolation without departing from the scope of the present invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.